NPAG2012 App1 Indep

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Energy Automation
Network Protection & Automation Guide 695
Terminology
APPENDIX
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Appendix 1 
Terminology
A1 A1A1 A1
Introduction
A A1 - 3
B A1 - 5
C A1 - 7 
D A1 - 10
E A1 - 13
F A1 - 15
G A1 - 16
H A1 - 17
I A1 - 18
K A1 - 20
L A1 - 21
M A1 - 22
N A1 - 24
O A1 - 25
P A1 - 27
R A1 - 30
S A1 - 33
T A1 - 36
U A1 - 37
V A1 - 38
X A1 - 38
Z A1 - 38
The introduction of computer technology means that the Protection 
Engineer must now be familiar with a range of technical terms in 
this field, in addition to the terms long associated with Protection 
and Control. Below is a list of terms and their meanings that are 
now commonly encountered in the Protection and Control field.
Contents
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Appendix 1 
Terminology
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A
AC Alternating Current
ACB Air Circuit Breaker
Accuracy The accuracy of a transducer is defined by 
the limits of intrinsic error and by the limits 
of variations.
Accuracy 
 class
A number used to indicate the accuracy 
range of a measurement transducer, 
according to a defined standard.
Active power 
(watt) transducer
A transducer used for the measurement of 
active electrical power
ADC Analogue to Digital Converter
A/D 
Conversion
The process of converting an analogue 
signal into an equivalent digital one, 
involving the use of an analogue to digital 
converter
Adjustment The operation intended to bring a 
transducer into a state of performance 
suitable for its use
AGC Automatic Gain Control
AI Analogue Input
AIS Air Insulated Switchgear
Alarm An alarm is any event (see below) tagged 
as an alarm during the configuration phase
All-or-nothing 
relay
An electrical relay which is intended to be 
energised by a quantity, whose value is 
either higher than that at which it picks up 
or lower than that at which it drops out
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Appendix 1 
Terminology
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A 
(cont.)
Anti-pumping 
device
A feature incorporated in a Circuit Breaker 
or reclosing scheme to prevent repeated 
operation where the closing impulse lasts 
longer than the sum of the relay and CB 
operating times
AO Analogue Output
AR Auto Reclose: A function associated with 
CB, implemented to carry out reclosure 
automatically to try to clear a transient fault
ARBITER Proprietary protocol for time 
synchronisation from ARBITER Systems, 
Inc. Paso Robles, California USA
Arcing time The time between instant of separation of 
the CB contacts and the instant of arc 
extinction
Auto-transformer A power transformer that does not provide 
galvanic isolation between primary and 
secondary windings
AUX Auxiliary
Auxiliary circuit A circuit which is usually energised by the 
auxiliary supply but is sometimes energised 
by the measured quantity
Auxiliary 
relay
An all-or-nothing relay energised via 
another relay, for example a measuring 
relay, for the purpose of providing higher 
rated contacts, or introducing a time delay, 
or providing multiple outputs from a single 
input.
Auxiliary 
supply
An a.c. or d.c. electrical supply other than 
the measured quantity which is necessary 
for the correct operation of the transducer
AVR Automatic Voltage Regulator
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Appendix 1 
Terminology
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B
Back-up protection A protection system intended to 
supplement the main protection in case the 
latter should be ineffective, or to deal with 
faults in those parts of the power system 
that are not readily included in the 
operating zones of the main protection
Bay Set of LV, MV, or HV plant and devices, 
usually controlled by a bay computer
BC Bay Computer. Computer dedicated to the 
control of one or several bays within a 
substation
BCD Binary Coded Decimal
BCP Bay Control Point. A local keypad at bay 
level to control the elements of a single bay
Biased 
relay
A relay in which the characteristics are 
modified by the introduction of some 
quantity other than the actuating quantity, 
and which is usually in opposition to the 
actuating quantity
Bias 
current
The current used as a bias quantity in a 
biased relay
BIOS Basic Input/Output System (of a computer 
or microprocessor)
BT Booster Transformer
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Appendix 1 
Terminology
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B 
(cont.)
Booster 
Transformer
A current transformer whose primary 
winding is in series with the catenary and 
secondary winding in the return conductor 
of a classically-fed a.c. overhead electrified 
railway. Used at intervals to ensure that 
stray traction return currents, with their 
potential to cause interference in nearby 
communication circuits, are minimised
Burden The loading imposed by the circuits of the 
relay on the energising power source or 
sources, expressed as the product of 
voltage and current (volt-amperes, or watts 
if d.c.) for a given condition, which may be 
either at ‘setting’ or at rated current or 
voltage. The rated output of measuring 
transformers, expressed in VA, is always at 
rated current or voltage and it is important, 
in assessing the burden imposed by a relay, 
to ensure that the value of burden at rated 
current is used
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Appendix 1 
Terminology
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C
C Capacitance
CAD Computer Aided Design
Calibration The set of operations which establish, 
under specified conditions, the relationship 
between values indicated by a transducer 
and the corresponding values of a quantity 
realized by a reference standard. (This 
should not be confused with ‘adjustment’, 
q.v.)
CB Circuit Breaker
CBC Compact Bay Controller. Small capacity 
bay computer for Medium Voltage 
applications
CBCT Core Balance Current Transformer
CCR Central Control Room
CDM Conceptual Data Modelling is an activity 
whose aims are:
• to define objects and links and naming 
conventions for their identifications
• to guarantee interoperability between 
subsystems
• to define standard exchange formats 
between system configurator and 
subsystem configurators
Characteristic 
angle
The angle between the vectors 
representing two of the energising 
quantities applied to a relay and used for 
the declaration of the performance of the 
relay
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Appendix 1 
Terminology
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C 
(cont.)
Characteristic 
curve
The curve showing the operating value of 
the characteristic quantity corresponding 
to various values or combinations of the 
energising quantities
Characteristic 
Impedance Ratio 
(C.I.R.)
The maximum value of the System 
Impedance Ratio up to which the relay 
performance remains within the prescribed 
limits of accuracy
Characteristic 
quantity
A quantity, the value of which characterises 
the operation of the relay, for example, 
current for an overcurrent relay, voltage for 
a voltage relay, phase angle for a directional 
relay, time for an independent time delay 
relay, impedance for an impedance relay
Check protection 
system
An auxiliary protection system intended to 
prevent tripping due to inadvertent 
operation of the main protection system
CHP Combined Heat and Power
Circuit insulation 
voltage
The highest circuit voltage to earth on 
which a circuit of a transducer may be used 
and which determines its voltage test
Class index The number which designates the accuracy 
class
Closing Impulse 
time
The time during which a closing impulse is 
given to the CB
Closing Time The time for a CB to close, from the time of 
energisation of the closing circuit to making 
of the CB contacts
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Terminology
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C 
(cont.)
Compliance voltage 
(accuracy limiting 
output voltage)
For current output signals only, the output 
voltage up to which the transducer meets 
its accuracy specification
Conjunctive test A test of a protection system including all 
relevant components and ancillary 
equipment appropriately interconnected. 
The test may be parametric or specific
Conversion 
coefficient
The relationship of the value of the 
measurand to the corresponding value of 
the output
Core Balance 
Current 
Transformer
A ring-type Current Transformer in which all 
primary conductors are passed through the 
aperture of the CBCT. Hence the 
secondary current is proportional only to 
any imbalance in current. Used for sensitive 
earth-fault protection
Counting Relay A relay that counts the number of times it is 
energised and actuates an output after a 
desired count has been reached.
CSV Character (or Comma) Separated Values 
format. A widely used format for the 
exchange of data between different 
software, in which the individual data items 
a separated by a known character – usually 
a comma
CT Current Transformer
Current 
transducer
A transducer used for the measurement of 
a.c. current
CVT Capacitor Voltage Transformer. A voltage 
transformer that uses capacitors to obtain a 
voltage divider effect. Used at EHV 
voltages instead of an electromagnetic VT 
for size/cost reasons
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Appendix 1 
Terminology
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D
DAC Digital to Analogue Converter
DAR 
Delayed auto-reclose
DAT 
Digital Audio Tape
DBMS 
Data Base Management system
DCF77 
LF transmitter located at Mainflingen, 
Germany, broadcasting a time signal on a 
77.5kHz frequency
DCP Device Control Point: local keypad on device 
level to control the switchgear, often 
combined with local/remote switch
DCS Distributed Control System
Dead Time (auto-
reclose)
The time between the fault arc being 
extinguished and the CB contacts re-
making
De-ionisation time 
(auto-reclose)
The time required for dispersion of ionised 
air after a fault is cleared so that the arc will 
not re-strike on re-energisation
Delayed Auto-
Reclose
An auto-reclosing scheme which has a time 
delay in excess of the minimum required for 
successful operation
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Appendix 1 
Terminology
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D 
(cont.)
Dependent time 
measuring relay
A measuring relay for which times depend, 
in a specified manner, on the value of the 
characteristic quantity
DFT Discrete Fourier Transform
Digital Signal 
Processor
A microprocessor optimised in both 
hardware architecture and software 
instruction set for the processing of 
analogue signals digitally, through use of 
the DFT and similar techniques
Digital Signal 
Processing
A technique for the processing of digital 
signals by various filter algorithms to obtain 
some desired characteristics in the output. 
The input signal to the processing 
algorithm is usually the digital 
representation of an analogue signal, 
obtained by A/D conversion
Directional relay A protection relay in which the tripping 
decision is dependent in part upon the 
direction in which the measured quantity is 
flowing
Discrimination The ability of a protection system to 
distinguish between power system 
conditions for which it is intended to 
operate and those for which it is not 
intended to operate
Distortion 
factor
The ratio of the r.m.s. value of the harmonic 
content to the r.m.s. value of the non-
sinusoidal quantity
DNP Distributed Network Protocol. A proprietary 
communication protocol used on 
secondary networks between HMI, 
substation computers or Bay Computers 
and protective devices
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Appendix 1 
Terminology
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D 
(cont.)
DOL Direct-on-Line
Direct-on-Line A method of motor starting, in which full 
line voltage is applied to a stationary motor
Drop-out 
 (or drop-off)
A relay drops out when it moves from the 
energised position to the un-energised 
position
Drop-out/pick-up 
ratio
The ratio of the limiting values of the 
characteristic quantity at which the relay 
resets and operates. This value is 
sometimes called the differential of the 
relay
DSP Digital Signal Processing
DT Definite time
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Appendix 1 
Terminology
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E
Earth fault 
protection system
A protection system which is designed to 
respond only to faults to earth
Earthing transformer A three-phase transformer intended 
essentially to provide a neutral point to a 
power system for the purpose of earthing
Effective range The range of values of the characteristic 
quantity or quantities, or of the energising 
quantities to which the relay will respond 
and satisfy the requirements concerning it, 
in particular those concerning precision
Effective setting The ‘setting’ of a protection system 
including the effects of current 
transformers. The effective setting can be 
expressed in terms of primary current or 
secondary current from the current 
transformers and is so designated as 
appropriate
Electrical relay A device designed to produce sudden 
predetermined changes in one or more 
electrical circuits after the appearance of 
certain conditions in the electrical circuit or 
circuits controlling it
NOTE: The term ‘relay’ includes all the 
ancillary equipment calibrated with the 
device
Electromechanical 
relay
An electrical relay in which the designed 
response is developed by the relative 
movement of mechanical elements under 
the action of a current in the input circuit
EMC Electro-Magnetic Compatibility
Embedded 
generation
Generation that is connected to a 
distribution system (possibly at LV instead 
of HV) and hence poses particular 
problems in respect of electrical protection
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Appendix 1 
Terminology
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E 
(cont.)
e.m.f. Electro-motive Force ( or voltage)
Energising quantity The electrical quantity, either current or 
voltage, which along or in combination with 
other energising quantities, must be 
applied to the relay to cause it to function
EPROM Electrically Programmable Read Only 
Memory
Error (of a 
transducer)
The actual value of the output minus the 
intended value of the output, expressed 
algebraically
Event An event is any information acquired or 
produced by the digital control system
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Appendix 1 
Terminology
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F
FAT Factory Acceptance Test. Validation 
procedures witnessed by the customer at 
the factory
Fault Passage 
Indicator
A sensor that detects the passage of 
current in excess of a set value (i.e. current 
due to a fault) at the location of the sensor. 
Hence, it indicates that the fault lies 
downstream of the sensor
FBD Functional Block Diagram: One of the IEC 
61131-3 programming languages
Fiducial value A clearly specified value to which reference 
is made in order to specify the accuracy of 
a transducer. (For transducers, the fiducial 
value is the span, except for transducers 
having a reversible and symmetrical output 
when the fiducial value may be either the 
span or half the span as specified by the 
manufacturer. It is still common practice, 
however, for statements of accuracy for 
frequency transducers to refer to ‘percent 
of centre-scale frequency’ and, for phase 
angle transducers, to an error in electrical 
degrees.)
FPI Fault Passage Indicator
Frequency 
transducer
A transducer used for the measurement of 
the frequency of an a.c. electrical quantity
Full duplex 
communications
A communications system in which data 
cantravel simultaneously in both directions
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Appendix 1 
Terminology
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G
Gateway The Gateway is a computer which provides 
interfaces between the local computer 
system and one or several SCADA (or RCC) 
systems
GIS Gas Insulated Switchgear (usually SF6)
Global Positioning 
System
A system used for locating objects on Earth 
precisely, using a system of satellites in 
geostationary orbit in Space. Used by some 
numerical relays to obtain accurate time 
information
GMT Greenwich Mean Time
GPS Global Positioning System
GTO Gate Turn-off Thyristor
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Appendix 1 
Terminology
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H
Half- duplex 
communications
A communications system in which data 
can travel in both directions, but only in one 
direction at a time
High-speed 
reclosing
A reclosing scheme where re-closure is 
carried out without any time delay other 
than that required for de-ionisation, etc.
HMI Human Machine Interface. The means by 
which a human inputs data to and receives 
data from a computer-based system. 
Usually takes the form of a Personal 
Computer (PC) (desktop or portable) with 
keyboard, screen and pointing device
HRC High Rupturing Capacity (applicable to 
fuses)
HSR High Speed Reclosing
HV High Voltage
HVDC High Voltage Direct Current
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Appendix 1 
Terminology
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I
I Current
ICCP Term used for IEC 60870-6-603 protocol
ICT Interposing Current Transformer (software 
implemented)
I.D.M.T. Inverse Definite Minimum Time
IGBT Insulated Gate Bipolar Transistor
I/O Input/Output
IED Intelligent Electronic Device. Equipment 
containing a microprocessor and software 
used to implement one or more functions 
in relation to an item of electrical 
equipment (e.g. a bay controller, remote 
SCADA interface/protocol converter). A 
microprocessor-based numerical relay is 
also an IED. IED is a generic term used to 
describe any microprocessor-based 
equipment, apart from a computer
IGBT Insulated Gate Bipolar Transistor
Independent time 
measuring relay
A measuring relay, the specified time for 
which can be considered as being 
independent, within specified limits, of the 
value of the characteristic quantity
Influence quantity A quantity which is not the subject of the 
measurement but which influences the 
value of the output signal for a constant 
value of the measurand
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Appendix 1 
Terminology
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I 
(cont.)
Input quantity The quantity, or one of the quantities, which 
constitute the signals received by the 
transducer from the measured system
Instantaneous 
relay
A relay that operates and resets with no 
intentional time delay.
 
NOTE: All relays require some time to 
operate; it is possible, within the above 
definition, to discuss the operating time 
characteristics of an instantaneous relay
Insulated Gate 
Bipolar Transistor
A special design of transistor that is suitable 
for handling high voltages and currents 
(relative to an ordinary transistor). 
Frequently used in static power control 
equipment (inverters, controlled rectifiers, 
etc) due to the flexibility of control of the 
output
Intrinsic error An error determined when the transducer is 
under reference conditions
Inverse time delay 
relay
A dependent time delay relay having an 
operating time which is an inverse function 
of the electrical characteristic quantity
Inverse time relay 
with definite 
minimum time 
(I.D.M.T.)
An inverse time relay having an operating 
time that tends towards a minimum value 
with increasing values of the electrical 
characteristic quantity
IRIG-B An international standard for time 
synchronisation
ISO International Standards Organisation
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Appendix 1 
Terminology
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K
K-bus 
(K-bus Courier)
Term used for the Courier protocol on 
K-Bus interface for K-Relay range 
manufactured by AREVA
Knee-point 
e.m.f.
That sinusoidal e.m.f. applied to the 
secondary terminals of a current 
transformer, which, when increased by 10%, 
causes the exciting current to increase by 
50%
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Appendix 1 
Terminology
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L
L Inductance
LAN Local Area Network
LCD Liquid Crystal Display
LED Light Emitting Diode
LD Ladder Diagram. One of the IEC 61131-3 
programming languages
LDC Line drop compensator
Limiting value of the 
output current
The upper limit of output current which 
cannot, by design, be exceeded under any 
conditions
Local Control Mode When set for a given control point it means 
that the commands can be issued from this 
point
Lock-out 
(auto-reclose)
Prevention of a CB reclosing after tripping
Long-term stability The stability over a period of one year
Low-speed 
auto-reclose
See Delayed Auto-Reclose
LV Low Voltage
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Terminology
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M
Main protection The protection system which is normally 
expected to operate in response to a fault 
in the protected zone
Maximum 
permissible values of 
the input current and 
voltage
Values of current and voltage assigned by 
the manufacturer which the transducer will 
withstand indefinitely without damage
MCB Miniature Circuit Breaker
MCCB Moulded Case Circuit Breaker
Mean-sensing 
transducer
A transducer which actually measures the 
mean (average) value of the input 
waveform but which is adjusted to give an 
output corresponding to the r.m.s. value of 
the input when that input is sinusoidal
Measurand A quantity subjected to measurement
Measuring element A unit or module of a transducer which 
converts the measurand, or part of the 
measurand, into a corresponding signal
Measuring range That part of the span where the 
performance complies with the accuracy 
requirements
Measuring relay An electrical relay intended to switch when 
its characteristic quantity, under specified 
conditions and with a specified accuracy 
attains its operating value
Metering 
(non-tariff)
Values computed depending on the values 
of digital or analogue inputs during variable 
periods
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Appendix 1 
Terminology
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M 
(cont.)
Metering 
(tariff)
Energy values computed from digital and/
or analogue inputs during variable periods 
and dedicated to energy measurement for 
billing (tariff) purposes
Mid Point Sectioning 
Substation
A substation located at the electrical 
interface of two sections of electrified 
railway. It contains provision for the 
coupling of the sections electrically in the 
event of loss of supply to one section
ModBus Proprietary communication protocol used 
on secondary networks between HMI, 
substation computers or Bay Computers 
and protective devices
MPSS Mid Point Sectioning Substation (electrified 
railways)
Multi-element 
transducer
A transducer having two or more 
measuring elements. The signals from the 
individual elements are combined to 
produce an output signal corresponding to 
the measurand
Multi-section 
transducer
A transducer having two or more 
independent measuring circuits for one or 
more functions
Multi-shot reclosing A reclosing scheme that permits more than 
one reclosing operation of a CB after a fault 
occurs before lock-out occurs
MV Medium Voltage
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Terminology
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N
N/C Normally Closed
N/O Normally Open
Nominal range of use A specified range of values which it isintended that an influence quantity can 
assume without the output signal of the 
transducer changing by amounts in excess 
of those specified
Notching relay A relay which switches in response to a 
specific number of applied impulses
NPS Negative Phase Sequence
NS Neutral Section (electrified railways)
Numerical relay A protection relay which utilises a Digital 
Signal Processor to execute the protection 
algorithms in software
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Appendix 1 
Terminology
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O
OCB Oil Circuit Breaker
Off-Load Tap 
Changer
A tap changer that is not designed for 
operation while the transformer is supplying 
load
OHL Overhead line
OLTC On Load Tap Changer.
On Load Tap 
Changer
A tap changer that can be operated while 
the transformer is supplying load.
Opening time The time between energisation of a CB trip 
coil and the instant of contact parting
Operating current 
(of a relay)
The current at which a relay will pick up
Operating time (CB) The time between energisation of a CB trip 
coil and arc extinction
Operating time 
(relay)
With a relay de-energised and in its initial 
condition, the time which elapses between 
the application of a characteristic quantity 
and the instant when the relay operates
Operating time 
characteristic
The curve depicting the relationship 
between different values of the 
characteristic quantity applied to a relay 
and the corresponding values of operating 
time
Operating value The limiting value of the characteristic 
quantity at which the relay actually operates
 
Optical Ground Wire – a ground wire that 
includes optical fibres to provide a 
communications link
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Terminology
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O 
(cont.)
OSI 7-layer model The Open Systems Interconnection 7-layer 
model is a model developed by ISO for 
modelling of a communications network.
Output common 
mode interference 
voltage
An unwanted alternating voltage which 
exists between each of the output terminals 
and a reference point
Output current (of a 
transducer)
The current produced by the transducer 
which is an analogue function of the 
measurand
Output load The total effective resistance of the circuits 
and apparatus connected externally across 
the output terminals
Output power (of a 
transducer)
The power available at the transducer 
output terminals
Output series mode 
interference voltage
An unwanted alternating voltage appearing 
in series between the output terminals and 
the load
Output signal An analogue or digital representation of the 
measurand
Output span (span) The algebraic difference between the lower 
and upper nominal values of the output 
signal
Overcurrent relay A protection relay whose tripping decision 
is related to the degree by which the 
measured current exceeds a set value.
Overshoot time The overshoot time is the difference 
between the operating time of the relay at a 
specified value of the input energising 
quantity and the maximum duration of the 
value of input energising quantity which, 
when suddenly reduced to a specific value 
below the operating level, is insufficient to 
cause operation
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P
Parametric 
conjunctive test
A conjunctive test that ascertains the range 
of values of each parameter for which the 
test meets specific performance 
requirements
PCB Printed Circuit Board
PCC Point of Common Coupling
PED Power Electronic Device
Phase angle 
transducer
A transducer used for the measurement of 
the phase angle between two a.c. electrical 
quantities having the same frequency
Pick-up A relay is said to ‘pick-up’ when it changes 
from the de-energised position to the 
energised position
Pilot channel A means of interconnection between 
relaying points for the purpose of 
protection
PLC Programmable Logic Controller. A 
specialised computer for implementing 
control sequences using software
PLCC Power Line Carrier Communication
Point of Common 
Coupling
The interface between an in-plant network 
containing embedded generation and the 
utility distribution network to which the 
in-plant network is connected
POW Point-on- Wave. Point-on-wave switching is 
the process to control moment of switching 
to minimise the effects (inrush currents, 
overvoltages)
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Appendix 1 
Terminology
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P 
(cont.)
Power Electronic 
Device
 An electronic device (e.g. thyristor or IGBT) 
or assembly of such devices (e.g. inverter). 
Typically used in a power transmission 
system to provide smooth control of output 
of an item of plant
Power factor The factor by which it is necessary to 
multiply the product of the voltage and 
current to obtain the active power
Power Line Carrier 
Communication
A mean of transmitting information over a 
power transmission line by using a cariier 
fraquency superimpozed on the normal 
power frequency.
PPS Positive Phase Sequence
Protected zone The portion of a power system protected 
by a given protection system or a part of 
that protection system
Protection 
equipment
The apparatus, including protection relays, 
transformers and ancillary equipment, for 
use in a protection system
Protection 
relay
A relay designed to initiate disconnection of 
a part of an electrical installation or to 
operate a warning signal, in the case of a 
fault or other abnormal condition in the 
installation. A protection relay may include 
more than one electrical element and 
accessories
Protection 
scheme
The co-ordinated arrangements for the 
protection of one or more elements of a 
power system. A protection scheme may 
comprise several protection systems
Protection 
system
A combination of protection equipment 
designed to secure, under predetermined 
conditions, usually abnormal, the 
disconnection of an element of a power 
system, or to give an alarm signal, or both
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Appendix 1 
Terminology
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P 
(cont.)
Protocol A set of rules that define the method in 
which a function is carried out – commonly 
used in respect of communications links, 
where it defines the hardware and software 
features necessary for successful 
communication between devices.
PSM Plug Setting Multiple – a term used in 
conjunction with electromechanical relays, 
denoting the ratio of the fault current to the 
current setting of the relay
PSTN Public Switched Telephone Network
PT100 Platinum resistance temperature probe
A1 - 30 NRJED311332EN
Appendix 1 
Terminology
A1 A1A1 A1
R
R Resistance
R.M.S.-sensing 
transducer
A transducer specifically designed to 
respond to the true r.m.s. value of the input 
and which is characterised by the 
manufacturer for use on a specified range 
of waveforms
Ratio correction A feature of digital/numerical relays that 
enables compensation to be carried out for 
a CT or VT ratio that is not ideal
Rating The nominal value of an energising quantity 
that appears in the designation of a relay. 
The nominal value usually corresponds to 
the CT and VT secondary ratings
RCD Residual Current Device. A protection 
device which is actuated by the residual 
current
RCP Remote Control Point. The Remote Control 
Point is a SCADA interface. Several RCP’s 
may be managed with different 
communication protocols. Physical 
connections are done at a Gateway or at 
substation computers or at a substation 
HMI
Reactive power (var) 
transducer
A transducer used forthe measurement of 
reactive electrical power
Reclaim time (auto-
reclose)
The time between a successful closing 
operation, measured from the time the 
auto-reclose relay closing contact makes 
until a further reclosing sequence is 
permitted in the event of a further fault 
occurring
SESA214753
Note
Marked définie par SESA214753
SESA214753
Note
Marked définie par SESA214753
A1 A1
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Appendix 1 
Terminology
A1 A1
R 
(cont.)
REF Restricted Earth Fault
Reference 
conditions
Conditions of use for a transducer 
prescribed for performance testing, or to 
ensure valid comparison of results of 
measurement
Reference 
range
A specified range of values of an influence 
quantity within which the transducer 
complies with the requirements concerning 
intrinsic errors
Reference 
value
A specified single value of an influence 
quantity at which the transducer complies 
with the requirements concerning intrinsic 
errors
Relay See Protection relay
Resetting 
value
The limiting value of the characteristic 
quantity at which the relay returns to its 
initial position
Residual current The algebraic sum, in a multi-phase system, 
of all the line currents
Residual voltage The algebraic sum, in a multi-phase system, 
of all the line-to-earth voltages
Response time The time from the instant of application of 
a specified change of the measurand until 
the output signal reaches and remains at its 
final steady value or within a specified band 
centred on this value
Reversible output 
current
An output current which reverses polarity in 
response to a change of sign or direction of 
the measurand
A1 - 32 NRJED311332EN
Appendix 1 
Terminology
A1 A1A1 A1
R 
(cont.)
Ripple content 
of the output
With steady-state input conditions, the 
peak-to-peak value of the fluctuating 
component of the output
r.m.s. Root Mean Square
RMU Ring Main Unit
ROCOF Rate Of Change Of Frequency (protection 
relay)
RSVC Relocatable Static Var Compensator
RTD Resistance Temperature Detector
RTOS Real Time Operating System
RTU Remote Terminal Unit. An IED used 
specifically for interfacing between a 
computer and other devices. Sometimes 
may include control/monitoring/storage 
functions
A1 A1
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Appendix 1 
Terminology
A1 A1
S
SAT Site Acceptance Test. Validation 
procedures for equipment executed with 
the customer on site
SCADA Supervisory Control and Data Acquisition
SCL Substation Configuration Language. 
Normalised configuration language for 
substation modelling (as expected by IEC 
61850-6)
SCP Substation Control Point. HMI computers at 
substation level allowing the operators to 
control the substation
SCS Substation Control System
Setting The limiting value of a ‘characteristic’ or 
‘energising’ quantity at which the relay is 
designed to operate under specified 
conditions. Such values are usually marked 
on the relay and may be expressed as direct 
values, percentages of rated values, or 
multiples
SFC Sequential Function Chart: One of the IEC 
61131-3 programming languages
Short-term stability The stability over a period of 24 hours
Simplex 
communications 
system
A communications system in which data 
can only travel in one direction
Single-shot reclosing An auto-reclose sequence that provides 
only one reclosing operation, lock-out of 
the CB occurring if it subsequently trips
S.I.R. System Impedance Ratio
Single element 
transducer
A transducer having one measuring 
element
A1 - 34 NRJED311332EN
Appendix 1 
Terminology
A1 A1A1 A1
S 
(cont.)
SOE Sequence Of Events
SOTF Switch on to Fault (protection)
Specific conjunctive 
test
A conjunctive test using specific values of 
each of the parameters
Spring winding time For spring-closed CB’s, the time for the 
spring to be fully charged after a closing 
operation
ST Structured Text: One of the IEC 61131-3 
programming languages
Stability (of a 
transducer)
The ability of a transducer to keep its 
performance characteristics unchanged 
during a specified time, all conditions 
remaining constant
Stability (of a 
protection system)
The quantity whereby a protection system 
remains inoperative under all conditions 
other than those for which it is specifically 
designed to operate
Stability limits (of a 
protection system)
The r.m.s. value of the symmetrical 
component of the through fault current up 
to which the protection system remains 
stable
Starting relay A unit relay which responds to abnormal 
conditions and initiates the operation of 
other elements of the protection system
STATCOM A particular type of Static Var Compensator, 
in which Power Electronic Devices such as 
GTO’s are used to generate the reactive 
power required, rather than capacitors and 
inductors
A1 A1
A1 - 35NRJED311332EN
Appendix 1 
Terminology
A1 A1
S 
(cont.)
Static relay An electrical relay in which the designed 
response is developed by electronic, 
magnetic, optical or other components 
without mechanical motion. Excludes 
relays using digital/numeric technology
Static Var 
Compensator
A device that supplies or consumes reactive 
power, comprised solely of static 
equipment. It is shunt-connected on 
transmission lines to provide reactive power 
compensation
STC Short Time Current (rating of a CT)
Storage conditions The conditions, defined by means of ranges 
of the influence quantities, such as 
temperature, or any special conditions, 
within which the transducer may be stored 
(non-operating) without damage
SVC Static Var Compensator
System disturbance 
time (auto-reclose)
The time between fault inception and CB 
contacts making on successful re-closure
System impedance 
ratio
The ratio of the power system source 
impedance to the impedance of the 
protected zone
A1 - 36 NRJED311332EN
Appendix 1 
Terminology
A1 A1A1 A1
T
T101 Term used for IEC 60870-5-101 protocol
Tap changer A mechanism, usually fitted to the primary 
winding of a transformer, to alter the turns 
ratio of the transformer by small discrete 
amounts over a defined range
TCP/IP Transmission Control Protocol/Internet 
Protocol. A common protocol for the 
transmission of messages over the Internet
TCS Trip Circuit Supervision
TC57 Technical Committee 57 working for the IEC 
and responsible for producing standards in 
the field of Protection (e.g. IEC 61850)
TF a) Transfer Function of a device 
 (usually an element of a control system
b) Transient Factor (of a CT)
Through fault 
current
The current flowing through a protected 
zone to a fault beyond that zone
Time delay A delay intentionally introduced into the 
operation of a relay system
Time delay relay A relay having an intentional delaying 
device
TPI Tap Position Indicator (for transformers)
Transducer 
(electrical measuring 
transducer)
A device that provides a d.c. output quantity 
having a definite relationship to the a.c. 
measurand
Transducer with 
offset zero (live zero)
A transducer which gives a predetermined 
output other than zero when the 
measurand is zero
Transducer with 
suppressed zero
A transducer whose output is zero when 
the measurand is less than a certain value
A1 A1
A1 - 37NRJED311332EN
Appendix 1 
Terminology
A1 A1
U
Unit electrical 
relay
A single relay that can be used alone or in 
combinations with others
Unit protection A protection system that is designed to 
operate only for abnormal conditions within 
a clearly defined zone of the power system
Unrestricted 
protection
A protection system which has no clearly 
defined zone of operation and which 
achieves selective operation onlyby time 
grading
UCA Utility Communications Architecture
UPS Uninterruptible Power Supply
UTC Universal Time Coordinates
A1 - 38 NRJED311332EN
Appendix 1 
Terminology
A1 A1
V / X / Z
V Voltage
VCB Vacuum Circuit Breaker
VDEW Term used for IEC 60870-5-103 protocol. 
The VDEW protocol is a subset of the IEC 
60870-5-103 protocol
Vector group 
compensation
A feature of digital and numerical relays 
that compensates for the phase angle shift 
that occurs in transformers (including VT’s) 
due to use of dissimilar winding 
connections – e.g. transformers connected 
delta/star
Voltage 
transducer
A transducer used for the measurement of 
a.c. voltage
VT Voltage Transformer
X Reactance
Z Impedance